I had that before, but unfortunately do not remember, where it comes from, it might have to do something with V2- Style Attachments coming from Domino Websites... Try Evaluate( @AttachmentNames ) to get a Variant containing the names of all attachments. Then loop through this with a Forall- loop and try the NotesDocument.getAttachment( strLoopValue ) - Function to get a handle to the attachment. For further info read here and follow the links on that page, especially this one
Code would be something like this:
Dim doc as NotesDocument
Dim varAttachmentNamens as Variant
Dim object as NotesEmbeddedObject
REM "Get the document here"
varAttachmentNames = Evaluate( "@AttachmentNames" , doc )
Forall strAttachmentName in varAttachmentNames
Set object = doc.GetAttachment( strAttachmentName )
REM "Do whatever you want..."
End Forall